In an increasing demand for high speed and high quantity radio communications, power saving in mobile stations is required. One of the techniques for the power saving is an intermittent reception.
The intermittent reception is a technique that achieves a power saving by activating a mobile station only when the mobile station needs to receive a signal transmitted from a base station while the mobile station is in a stand-by state. The intermittent reception is also referred to as a DRX (Discontinuous Reception), and further referred to as a DRX/DTX (Discontinuous Reception/Discontinuous Transmission) when the intermittent reception is accompanied with intermittent transmission of feedback information or the like.
In a downlink of Evolved UTRA and UTRAN (Super 3G) radio access schemes, which is in process of being standardized with HSDPA (High Speed Downlink Packet Access) or 3GPP, multiple access is achieved in such a manner that a scheduler of a base station controls transmission assignment by using a shared data channel in a time-sharing manner. The minimum time unit of a transmission assignment control is referred to as a sub-frame.
Data is transmitted via a shared data channel (DL-SCH) and is accompanied with a shared control channel (DL-SCCH). A signaling is performed on the DL-SCCH, for information identifying the user to which the data on the DL-SCH is addressed, or for information identifying a transport format (for example, a modulation method, coding ratio, or the like) used for transmitting the data on the DL-SCH.
Each mobile stations receives the DL-SCCH for every sub-frames, detects a presence/absence of the data addressed to the own mobile station. When the data addressed to the mobile station exist, the mobile station demodulates the DL-SCH and receives the data.
Here, for example, in a communication service such as Web Browsing, transmission data arrives at a system intermittently. Accordingly, when the mobile station receives the DL-SCCH at all times, a battery of the mobile station is exhausted.
In this regard, when data does not exist for a certain period of time, the battery of the mobile station can be saved by performing the DRX.
FIG. 1 shows an operational example of an intermittent reception in the mobile station when the DL-SCH and the DL-SCCH are used.
The mobile station checks the DL-SCCH to detect whether or not data addressed to the mobile station exists. When the data exists, the mobile station demodulates the DL-SCH.
In addition, when the data addressed to the mobile station exists, the mobile station continuously receives the DL-SCCH in the next and subsequent sub-frames (mode 1).
However, when the data addressed to the mobile station does not exist continuously for a predetermined time (t1), the mobile station shifts to a state where the DL-SCCH is intermittently received (for example, once in 8 sub-frames) (mode 2).
When the data addressed to the mobile station does not exist for a continuous predetermined time even after the mobile station shifts to the mode 2, the mobile station may further extend an interval of processing the DL-SCCH (for example, once in 16 sub-frames) (mode 3).
Whether or not to perform such stepwise DRX may be determined by following protocols which are arranged in advance between the base station and the mobile station.
When new data arrives at the base station while the mobile station performs the intermittent reception, the base station transmits data via the DL-SCH in accordance with the timing of the intermittent reception of the mobile station, and signals the arrival of the new data, via the DL-SCCH.
When the mobile station confirms that the data addressed to the own mobile station exists by processing the DL-SCCH during the intermittent reception, the mobile station returns to the mode of continuously processing DL-SCCH in the next and subsequent sub-frames (mode 1).
In FIG. 1, the mobile station in mode 1 reports a CQI (Channel Quality Indicator) to the base station in order to perform a scheduling or a link adaptation in the base station.
Here, the CQI is referred to as, for example, an index showing a radio channel quality such as a reception SIR (Signal to Interference Power Ratio) of a downlink pilot channel.
The base station compares the CQI among the users, and assigns a transmission opportunity to a user having a better radio quality (scheduling), so as to obtain an effect of multiuser-diversity.
Further, the base station can determine a transport format, a transmission power, or the like in accordance with the CQI. (link adaptation)
Meanwhile, the mobile station in mode 2 can report the CQI in accordance with the timing of the DRX (for example, right before the DRX). In other words, the CQI can be intermittently reported in accordance with DRX cycles (DTX).
When this DRX/DTX cycle is sufficiently short, a synchronization of the uplink signal is maintained between the base station and the mobile station, by periodically reporting the CQI.
However, when the DRX/DTX cycle is long, the uplink synchronization cannot be maintained even when the CQI is periodically reported. For example, when the DRX/DTX cycle is long as in mode 3, the uplink synchronization is lost even when the CQI is reported.
In a system in which uplink time division multiple access is performed, as in the case of Super 3G, reception timings in the base station are required to be within a predetermined time (for example, within a cyclic prefix of OFDM symbol).
As shown in FIG. 2, when the mobile station moves (T1 to T2), a distance between the mobile station (UE) and the base station (eNodeB) is changed, and thus a propagation delay is changed accordingly.
Therefore, when the mobile station transmits the CQI at certain intervals, the reception timing may not be synchronized and thereby causes interferences in the previous and next sub-frames.
In order to prevent such a synchronization loss, the base station is required to control the transmission timing in the mobile station, by measuring the reception timing in the base station so as to notify the measured reception timing to the mobile station.
Here, in order to simplify the description, it is assumed that mode 2 is referred to as a state where the reception timing in the base station will surely fall within a predetermined time when the CQI is reported in accordance with the DRX/DTX cycles, and where a uplink synchronization can be maintained by measuring the timing loss in the base station and by controlling the transmission timing in the mobile station.
Meanwhile, it is assumed that mode 3 is referred to as a state where the reception timing in the base station does not fall within a predetermined time and where interference is caused in the previous and next sub-frames when the CQI is reported in accordance with the DRX/DTX cycles.
Here, a problem arises in the above-described mode 3 in which the interference is caused in the previous and next sub-frames because of the synchronization loss when the CQI is reported in accordance with the DRX/DTX cycle.
Further, another problem arises in which the battery of the mobile station and the uplink radio resource are consumed because the CQI is periodically reported even when the data hardly arrives.
Meanwhile, in order to solve these problems, the CQI may be set not to be reported in accordance with the DRX/DTX cycles. In this case, however, another problem arises in which the link adaptation cannot be applied to the transmission of first data when downlink data is generated during the DRX, thereby a radio transmission efficiency is deteriorated.
Furthermore, when the first data is transmitted at the DRX timing before the uplink synchronization is established, a reception response transmitted from the mobile station to the first data (for example, ACK/NACK response of HARQ) may be lost because the base station cannot receive the reception response at a correct timing.
When the reception response is lost, a problem arises in which the first transmission is wasted and thereby a valuable radio resource is wasted.    Non-patent document 1: W-CDMA mobile communication method, under the editorship of Keiji Tachikawa, 4th printing, issued on Mar. 15, 2002, on page 222 to 223.